Heretofore, polarizing plates are used in optical devices such as liquid-crystal display devices, organic EL display devices, glasses and others, and the polarizing plates for such applications are generally constructed by sticking, with an adhesive, protective films on both surfaces of a polarizer formed of an iodine-colored, uniaxial-oriented polyvinyl alcohol resin film, for improving the strength, the water resistance and the moisture resistance of the polarizer.
For the protective film for such polarizing plates, used is an acetyl cellulose resin film (TAC film) of good optical transparency; and a hydrophilic adhesive is used for them, taking the matter into consideration that both the polarizer and the protective film are hydrophilic.
In the above-mentioned polarizer, the coloring polyiodide ions (e.g., I3−, I5−) exhibits the polarizability thereof as a result of the uniaxial orientation of the polyvinyl alcohol resin film. Therefore, when the polarizer receives moisture (water vapor), then the polyiodide therein decomposes into iodide ions (I−) and the coloration by the polyiodide ions is thereby reduced. This phenomenon is more remarkable in high-temperature environments. As a result, it is considered that the transmittance of the polarizer may increase and the polarizer may lose its polarizability. Accordingly, protective films for polarizing plates are required to have the ability to protect polarizers from the influence of external moisture and others thereon.
However, taking the matter into consideration that both the TAC film and the adhesive in conventional polarizing plates are hydrophilic, the thickness of the TAC film has heretofore made to be at least about 80 μm in order that the film may protect polarizers from the influence of external moisture thereon. Accordingly, conventional polarizing plates are problematic in that they could not satisfy the requirement in the recent art of optical display devices that the protective films for polarizing plates for such devices are thinned as much as possible (for example, their thickness is reduced to at most 40 μm).